1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device.
2. Description of Related Art
Active matrix liquid crystal display devices are known as being effective for high-quality display. They are constructed such that thin-film transistors are formed on a transparent substrate (usually a glass or quartz substrate) for respective pixels. Each thin-film transistor controls charge that enters or exits from an electrode (pixel electrode) of the associated pixel. The active matrix liquid crystal display devices require circuits (peripheral circuits) for driving the thin-film transistors for the respective pixels. In general, the peripheral circuits are constructed as an external IC circuit called a driver IC.
In an advanced version, the peripheral circuits formed by using thin-film transistors are integrated on the substrate. Providing a unified structure in which the pixel region and the peripheral circuit regions are integrated on the same substrate, this configuration facilitates the use of a liquid crystal panel.
As an example of application of the above liquid crystal panel, a projection-type liquid crystal display apparatus will be described below.
A first method of performing color display is to form color filters of R (red), G (green), and B (blue) in a liquid crystal panel. A second method is to prepare a plurality of panels and combine images formed by those panels. In recent years, with an increasing need for large screen display, the second method is used more frequently to implement a projection-type display apparatus, because in the first method the substrate size needs to be increased and hence it is difficult to manufacture a panel. The second method is disclosed in Japanese Utility Model Laid-Open No. 58-111580.
In the second method, to combine images, the consistency of optical axes is important. Conventionally, liquid crystal panels are arranged independently and the modulating of optical axes is performed by adjusting the position and orientation of each panel in a subtle manner. However, this is not preferable because it causes a cost increase and complicates the structure of the apparatus. There is known a further technique in which the same images are superimposed on each other to increase the screen size or the brightness. However, this technique has a problem of cost increase because it complicates the apparatus structure.
To solve the above problems, attempts have been made to integrate the three panels into a single panel. In this case, it is basically sufficient to generate a set of images corresponding to three colors of R, G and B. The brightness can be increased by generating two or more sets of images corresponding to R, G and B.
In this type of configuration, in forming peripheral driver circuit regions, it has been attempted to locate peripheral circuits that should be integrated at a high density at positions as close to the center of a substrate as possible, to increase a final production yield.
However, the above conventional liquid crystal display devices have two problems described below.
The first problem is as follows. A black matrix which is made of a reflective metal such as Cr and occupies a large area of a display screen is formed on the inside surface of a upper transparent glass substrate that is located on the display screen side. External light is reflected by the black matrix and comes out of the display screen. This lowers the contrast of a displayed image and hence makes it less visible, that is, lowers the display quality.
The second problem relates to a case where a black matrix is formed on an opposed substrate. In this case, as shown in FIG. 11A, a black matrix 1 is so formed as to overlap with ITO pixel electrodes 2 by 5-7 xcexcm in consideration of the bonding accuracy of the TFT substrate and the opposed substrate. Thus, the size of opening portions is restricted. In this case, to increase the brightness of the display device, it is necessary to employ a brighter back light, resulting in an increase in power consumption.
FIG. 11A shows how the black matrix 1 on the opposed substrate and the ITO pixel electrodes 2 overlap with each other.
Reference numerals 3-5 denote a signal line, a TFT, and a scanning line, respectively.
To solve the above two problems, an object of the present invention is to form a black matrix on TFTs of a driver circuit. This configuration has an advantage that the overlapping width can be reduced to about 2 xcexcm because of improved bonding accuracy that is obtained by forming the black matrix and the ITO pixel electrodes on the same substrate.
This advantage will be described with reference to 11B. FIG. 11B shows how they overlap with each other in a case where the black matrix 1 is formed on the TFT substrate. While in the former case (FIG. 11A) the aperture ratio is about 15% (overlapping width: 7 xcexcm), in the latter case (FIG. 11B) it is greatly increased to about 40% (overlapping width: 2 xcexcm).
On the other hand, in the above-mentioned configuration in which the opposed substrate is made large enough to be opposed to the driver circuits and the driver circuits are provided in the liquid crystal region, the driver circuit regions and the pixel region come close to each other, which requires light shielding even in the driver circuit regions.
Where the black matrix for light shielding of the pixel region is formed on the substrate on which TFTs are formed and is also used for light shielding of the driver circuits to satisfy the above requirement, there has occurred a problem that the capacitance of an interlayer insulating film between TFTs of the driver circuits and the black matrix is not negligible though the shielding itself does not cause any problem.
If the interlayer insulating film is a 3,000-xc3x85-thick silicon nitride film, it has a unit area capacitance of 2.50xc3x9710xe2x88x9216 F/xcexcm2. For example, if a clock line or the like of a driver circuit has a wiring line of 100 m in width and 50,000 xcexcm in length, a capacitance formed by this wiring line of the driver circuit and the black matrix amounts to 1.25xc3x9710xe2x88x929 F. In this case, if it is assumed that the wiring line of the driver circuit has a sheet resistance of 0.2 xcexa9/xcexcm2, its delay time amounts to 1.25xc3x9710xe2x88x927 sec, which will cause a problem when the wiring line is driven at several megahertz. The circuit characteristics are more important in the driver circuits than in the pixel TFTs. Therefore, it is necessary to reduce the capacitance of the interlayer insulating film formed between TFTs of the driver circuits and the black matrix.
It is practiced to form only a black matrix 16 for a pixel region 14 on a TFT substrate 11 so as to be adjacent to ITO electrodes 17 and form a black matrix 18 for driver circuit regions 13 on an opposed substrate 12, as shown in FIG. 12. However, although this configuration increases the aperture ratio, the number of manufacturing steps increases because of the need of forming the black matrix 16 and 18 on both of the TFT substrate 11 and the opposed substrate 12. In FIG. 12, reference numerals 15 and 19 respectively denote an aluminum wiring line and color filters of R, G and B.
It is now desired to provide a liquid crystal display device which enables light shielding of driver circuit regions without increasing the number of manufacturing steps.
Another object of the invention is to prevent a capacitance from occurring in an interlayer insulating film formed between TFTs of a driver circuit and a black matrix, to reduce, in turn, the delay time of the driver circuit, to thereby produce high-resolution images.
To attain the above objects, according to the invention, there is provided an active matrix liquid crystal display device comprising: a first insulating substrate comprising: a pixel region in which a plurality of pixels having respective thin-film transistors are arranged in matrix form; a driver circuit region for driving the pixel region, the driver circuit region being provided on the same surface as the pixel region and having thin-film transistors; and a black matrix formed over the driver circuit region; a second insulating substrate opposed to the first insulating substrate; and a liquid crystal material interposed between the first and second insulating substrates.
There is also provided an active matrix liquid crystal display device comprising: a first insulating substrate comprising: a pixel region in which a plurality of pixels having respective thin-film transistors are arranged in matrix form and a planation film is formed; a driver circuit region for driving the pixel region, the driver circuit region being provided on the same surface as the pixel region and having thin-film transistors; and a black matrix formed over the first insulating substrate a second insulating substrate opposed to the first insulating substrate: and a liquid crystal material interposed between the first and second insulating substrates.
Further, there is provided a liquid crystal display device comprising: a pair of transparent substrates; a liquid crystal interposed between the pair of transparent substrates; 2n liquid crystal panels that are constituted by using the pair of transparent substrates, where n is a natural number, the 2n liquid crystal panels comprising: active matrix pixel regions; driver circuits arranged around the pixel regions; and a black matrix formed over the first insulating substrate; and means for combining images produced by the 2n liquid crystal panels.
Still further, there is provided a liquid crystal display device comprising: a pair of transparent substrates; a liquid crystal interposed between the pair of transparent substrates; 2n liquid crystal panels that are constituted by using the pair of transparent substrates, where n is a natural number, the 2n liquid crystal panels comprising: active matrix pixel regions each having a planation film; driver circuits arranged around the pixel regions, one side of each of the driver circuits being adjacent to one of the pixel regions, and the other side being adjacent to the other pixel regions or the other driver circuits; and a black matrix formed over the first insulating substrate; and means for combining images produced by the 2n liquid crystal panels.
In the invention, the insulating substrate means a substrate made of a transparent material that has a certain level of strength with respect to external force, for instance, an inorganic material such as glass or quartz.
Where thin-film transistors (hereinafter called TFTs) are formed on a substrate, it is preferred to use a no-alkali glass substrate or a quartz substrate. Where it is intended to reduce the weight of a liquid crystal panel, there may be used a film that is low in birefringence, such as PES (polyethylene sulfate).
A TFT that is formed for each pixel or a peripheral driver circuit may be of a type in which the active layer is made of amorphous silicon or polysilicon.
An ITO (alloy of indium oxide and tin) transparent electrodes are formed on a substrate as electrodes for driving a liquid crystal material. In view of the heat resistance, it is desired to form a black matrix after formation of the ITO electrodes.
To prevent contrast reduction due to irregular reflection within the liquid crystal display device, the black matrix used in the invention may be of a type in which a black material is dispersed in a transparent material. Examples of the transparent material are inorganic materials such as glass and quartz and organic materials such as resin. From the viewpoint of easiness of manufacture, resin materials such as acrylic materials are preferred.
Examples of the black material are carbon black and a pigment. For examples, there may be used organic pigments of phthalocyanine pigments, quinacridon pigments, isoindolinone pigments, azo pigments, anthraquinone pigments, and dioxazine pigments.
Another method of forming the black matrix is to photosensitize a natural polymeric material such as gelatin, or a synthetic polymeric material such as polyvinyl alcohol, or polyvinyl pyrrolidone acrylic resin by a bichromate, then form a fine pattern by a photolithographic process, and finally dye it with an acid dye or a reactive dye.
A further method is to disperse a pigment such as carbon in a photosensitive resin such as a PVA resin, an acrylic resin, or a polyimide resin, and then form a fine pattern by a photolithographic process.
Among the above processes, the method of dispersing carbon black in an acrylic resin is preferred because it can reduce the resistance and form a thin film.
The method of dispersing a black material in a resin material may be selected properly in accordance with the black material used, from a stirring method using a stirrer, a ball mill method, three-roll method, etc. The dispersiveness of the black material can be improved by adding a small amount of dispersing agent such as a surfactant during a dispersing operation. To stabilize the dispersion and form a thin black matrix layer, it is desired that the average particle diameter of the black material be about 0.1 xcexcm. If the average particle diameter is larger than this value, there may occur color unevenness and hence the black matrix does not accomplish the intended function.
A black matrix can be formed on a TFT substrate in a manner similar to the manner of forming a resist pattern by an ordinary photolithographic method. That is, an organic solution in which a black material is dispersed is applied to a TFT substrate by spin coating or printing, then patterned by a known photographic method, and finally subjected to post-baking of about 200xc2x0 C.
The second insulating substrate that is opposed to the substrate on which TFTs are formed made of the same material as the latter. In addition to a transparent electrode, a member such as color filters, a black matrix, and/or a planation film may be formed on the opposed substrate when necessary. Where color filters are formed, first a black matrix is formed on the substrate, then color filters are formed, a planation film is then formed to flatten the uneven surface, and finally a transparent electrode layer is formed.
The liquid crystal material may be a nematic, cholesteric, or smectic material, or a dispersive liquid crystal in which one of those materials is dispersed in a transparent resin material. In particular, because the dispersive liquid crystal does not require the use of a polarizing plate, it can provide a bright panel.
Where a nematic, cholesteric, or smectic liquid crystal material is used, an orientation treatment is performed on one or both of the opposed surface of the pair of substrates to orient the liquid crystal material in a certain direction. The orientation treatment is actually a rubbing treatment in which the substrate surface is rubbed with a cloth or the like directly or through a thin film of an organic or inorganic material formed on one or both of the substrates.
The substrates that have been subjected to the orientation treatment are so disposed that the orientation-treated surfaces or the surfaces on which TFTs, transparent electrodes, etc. are formed are opposed to each other, and a liquid crystal material is interposed between the opposed substrates. Spacers or the like are distributed between the pair of substrates to provide a constant substrate gap. Spacers having a diameter of 1-10 xcexcm are used. The pair of substrate are fixed to each other with an epoxy adhesive, for instance. The adhesive is applied to a circumferential portion of the substrates so as to surround the pixel region and the peripheral driver circuit regions.